Method and apparatus for improving the efficiency of purification and deposition of polycrystalline silicon

ABSTRACT

Methods and apparatus for the commercial-scale production of purified polycrystalline silicon granules with one or more tailored levels of n- and p-type impurities from an impure silicon source such as, for example, metallurgical-grade silicon. Purification systems and methods involve: (1) one or more series of temperature controlled reactors or vessels provided with dual fluidized beds wherein solids and gases are transported so that varying degrees of purification and deposition of solid silicon is accomplished by strict control of temperature and residence time; (2) separation and recovery of the compounds of high-melting-point impurities such as, for example, FeSi and FeI 2 ; (3) purification, separation, and recycling of silicon tetraiodide; (4) separation and recovery of iodide compounds of lower-boiling-point liquid impurities such as for example, AlI 3 , in a continuous fractional distillation column, facilitated by an iodine reflux; (5) separation and recovery of very fine solid particles including impurity iodides and elemental silicon in a liquid mixture downstream of a fractional distillation column; (6) recovery of input iodine from the oxidation of both solid and liquid iodide impurity waste streams from the process.

CROSS-REFERENCE

This application claims the benefit of priority to U.S. ProvisionalApplication No. 60/838,479, filed Aug. 18, 2006, which is incorporatedby reference herein in its entirety.

FIELD OF INVENTION

The present invention relates to methods of producing silicon feedstock,and more specifically, to purifying impure silicon, by means of iodinechemical vapor transport to produce pure silicon feedstock for use infabricating photovoltaic and other semiconductor devices.

BACKGROUND OF INVENTION

The market demand for solar energy collection systems in the form ofphotovoltaic cells (PV) is growing in excess of 25% per year globallydue to factors including higher oil prices and government policiesaddressing such environmental issues as global warming. The dominantsubstrate material for PV is silicon, which accounts for about 90% ofinstalled commercial units at the present time. A serious shortcoming inthe silicon-based PV value chain, however, is that there is presently nodirect method of producing PV-grade polycrystalline silicon (PV-Si) atcompetitive prices. The main reason for this situation is that,historically, the PV industry relied mostly on scrap silicon materialthat was recycled from the microelectronics industry. Recently, theglobal demand for PV-Si has outstripped the supply of recycledelectronic-grade silicon (REG-Si) and the expectation is that thissource of silicon will no longer be able to meet the demand from the PVindustry.

Many PV manufacturers are now considering direct purchase ofelectronic-grade silicon (EG-Si), which is also in tight supply, butwhose price is as much as 10 times higher than the historical averageprice of REG-Si. The higher price of EG-Si is mainly due to thecomplexity and high capital cost of the trichlorosilane and silaneprocesses that dominate this industry at the present time. In manycases, the EG-Si producers are also forward integrated into themicroelectronics value chain and so these processes are optimized forthat end-user market. What is required for the PV industry is a processthat is simpler, more economical, and safer to operate than the dominantEG-Si processes.

In U.S. Pat. Nos. 6,712,908 and 6,468,886, Wang et al. disclose athree-step process for the production of PV- and EG-silicon. In thefirst step, impure metallurgical-grade silicon (MG-Si) is reacted withiodine at a temperature, (T<900° C.), which favors the formation ofsilicon tetraiodide, SiI₄. Sufficient SiI₄ is then produced in thisfashion to fill a holding tank. Once the required amount of SiI₄ isproduced it is then recycled to the initial reactor stage where thetemperature has been increased to above 1200° C. and the SiI₄ reactswith MG-Si to produce substantial quantities of an unstable silicondiiodide vapor compound, SiI₂. The SiI₂ is transported by naturalconvection to a cooler region of a “cold-wall” reactor where itdecomposes and deposits as polycrystalline silicon on solid substratesthat can be inert or high-purity silicon rods.

However, there exist a number of problems associated with the teachingsof this invention and others in the prior art, that, taken together,prevent the realization of a scalable and economical method for theproduction of PV- and EG-silicon. These shortcomings are described indetail below.

1. The use of a “cold-wall” vessel for the reaction and deposition ofsilicon is critical to the method and apparatus of the inventiondisclosed by Wang et al. Yet this leads to poor control of the spatialdistribution of silicon deposition due to three factors: 1) the SiI₂decomposition reaction that forms silicon is a function of temperature;2) SiI₂ readily decomposes to form solid silicon in the vapor phasewithout the requirement of a solid substrate; 3) the temperaturegradient between the SiI₂ formation zone (i.e., T˜1200° C.) and wallregion (i.e., T=200-700° C.) of the reactor is at least 500° C.Furthermore, as the product vapors that are saturated with SiI₂ form inthe reactor bottom, some of the vapor travels toward the cooler wallsand thereby creates a thermodynamic driving force for fine siliconpowder nucleation within the vapor phase. The quantity of silicon powdermay be anywhere from 10-50% of the total silicon produced at any giventime. This silicon powder will be produced homogeneously and will beentrained along with the liquid silicon tetraiodide, SiI₄, stream as itis injected into a batch distillation column. As there are no provisionsfor the separation of the entrained silicon fines, the distillationcolumn operation will be compromised and the process will need to beshutdown for frequent cleaning thereby making the process less viable.Also, the silicon thus produced is very fine and not generally in ausable form due to its tendency to oxidize with air at ambientconditions in the facilities of the end-user ingot and wafermanufacturers. To summarize, the interaction of the three factorsdescribed above results in the production of a substantial amount ofsilicon product that is both unsuitable for sale and difficult to removefrom the process, thereby reducing the economic viability of Wang et al.

2. More than half of the weight of impurities in MG-Si typicallyconsists of Fe atoms. While Fe reacts with SiI₄ to form FeI₂ vapor inthe lower part of the cold-wall chamber at temperatures of about 1250°C., as the vapor temperature decreases to 700-800° C. near thecold-wall, the Fe is converted to solid FeSi. Due to the poor control oftemperature in the cold-wall reactor, it is likely that a majority of Featoms will be entrained as solids within the liquid stream of silicontetraiodide as it is directed to the distillation unit.

Again, as in the case of gas-phase silicon formation, these impuritiesaffect the operation of the distillation column by contaminating therecycle stream and plugging of distillation column internals. As theteachings of Wang et al. do not accommodate the removal of theseimpurities, they tend to build up in the process and will be recycledback into the cold-wall reactor where they substantially reduce theoverall efficiency of purification.

3. The cold-wall reactor is operated as a natural-convection drivensystem and this leads to the formation of a vapor cloud located near theuppermost region of the reactor. Because of the existence of this vaporcloud, the preferential removal of Boron (B) and Phosphorous (P) on thetop section of the reactor does not occur as there is no provision madefor preferentially removing the iodides of these elements from the otherpredominant compounds in the vapor cloud such as silicon tetraiodide,iodine, and other impurities. Also, any elemental silicon or siliconiodide that is inadvertently removed from this section of the reactor isnot recoverable by the teachings of Wang et al.

4. Wang et al. teaches a method and apparatus for purifying silicontetraiodide in a distillation column that is operated in a batch modewith the input SiI₄ stream introduced in the bottom section. This typeof system is referred to as a “batch distillation without reflux”. Inthis mode of operation, the level of purification is generally not verygood and certainly cannot meet the 10,000-to-1 or more reduction ofimpurities levels in SiI₄ required for the process to be effective inthe recycle loop. Furthermore, large-scale use of batch distillation isnot generally practiced because of the high costs associated withstartup and shutdown operations.

5. Iodine raw material added to the process is typically more expensivethan the MG-Si. Therefore, the need to minimize the use of iodine withinthe process and to recover iodine from impurity output streams is animportant part of ensuring an economically scalable process. The methodand apparatus of Wang et al. does not teach how to recover iodine fromthe solid and liquid iodides formed (e.g., FeI₂ and AlI₃). Furthermore,Wang et al. does not show how to minimize the use of iodine within theprocess to minimize initial capital and operating costs for thecommercial plant.

6. The method and apparatus of Wang et al. assumes that there is nofree-iodine (i.e., I or I₂) left in the system once the second stage ofoperation is started and silicon tetraiodide is recycled into thecold-wall reactor. Thermodynamic calculations reveal, however, thatbetween 1100-1300° C. the reaction between solid Si and SiI₄ vapor inthe reactor bottom produces the following compounds with thestoichiometry indicated:Si_((s))+2.5SiI₄

3.4SiI₂+2.3I+0.3I₂+0.1SiI₃By neglecting to account for the presence of free iodine, thedistillation column design ignores the need to condense, purify, andrecycle this expensive raw material as there is no reflux capability onthe top of the column.

7. In a commercial process, the iodine raw material will containimpurities that need to be removed. If the source of the iodine is acaliche ore deposit then these impurities are generally water,non-volatile solids, and chloride-bromide compounds. No means to removethese impurities is disclosed in Wang et al.

8. Wang et al. does not provide an economical method for producingEG-Si. Experimental results provided by the teachings, for example,indicate a purity level for B and P of 4 and 7 ppm atomic, respectively,for the case where there is no recycling of purified SiI₄. In order todecrease the B and P levels even further to the EG-Si specificationsthat are in parts per billion will require a recycle ratio ofSiI₄-to-input MG-Si that is in the range of 100-1,000. This amount ofrecycling is prohibitively expensive in commercial systems and so amethod is needed to substantially reduce the SiI₄ recycle ratio and thesize of the distillation column to make this chemistry economical versusthe competing trichlorosilane and silane methods previously discussed.

9. Natural convection is the primary mode of mass transport in the“cold-wall” reactor. This method of mixing reactants does not lead tohigh productivity and is generally avoided in chemical process systemsin commercial applications because it leads to unnecessarily highcapital costs for plant and equipment.

10. There is no means to remove liquid iodide impurities in the batchdistillation column that have higher boiling points than CI₄.

In sum, the deficiencies in the forgoing invention make it verydifficult to economically produce purified silicon on acommercial-scale.

Other related art includes: U.S. Pat. No. 3,006,737 to Moates et al;U.S. Pat. No. 3,620,129 to Herrick; U.S. Pat. No. 4,910,163 to Jain; andU.S. Pat. No. 6,281,098 to Wang et al.

Related publications include: Herrick, C. S. et al., “High-puritySilicon from an Iodide Process Pilot Plant,” J. Electrochem. Soc., Vol.107, No. 2, February 1960, pp. 111-117; Glang, R. et al., “Silicon”, inThe Art and Science of Growing Crystals, John Wiley and Sons, New York,1963, pp. 80-87; Szekely, G., “Preparation of Pure Silicon by HydrogenReduction of Silicon Tetraiodide,” J. Electrochem. Soc., Vol. 104, No.11, November 1957, pp. 663-667; Litton, F. B., et al., “High PuritySilicon,” J. Electrochem. Soc., Vol. 101, No. 6, June 1954, pp. 287-292;Glang R., et al., “Impurity Introduction during Epitaxial Growth ofSilicon,” IBM Journal, July 1960, pp. 299-301; and Hillel, R. et al.,“Stabilité Thermique et Propriétés Thermodynamiques des lodures dePhosphore a l'état Condensé et Gaseux,” J. Chimie Physique, Vol. 73, No.9-10, 1976, pp. 845-848.

SUMMARY OF INVENTION

Accordingly, the invention provides methods and systems for producing orpurifying silicon for many commercial applications from a variety ofsource materials. The present invention further provides processes thatare scalable to commercial capacity (i.e., 500-5,000 tonne per year) forproducing PV- and EG-grade silicon from an impure silicon source such asmetallurgical- or chemical-grade silicon (typically 98-99.5% puresilicon).

Another aspect of the present invention also provides economical, highthrough-put methods of depositing pure silicon granules which are usefulfor applications in the continuous processes of leading PV manufacturersusing string ribbon or spherical cells.

The present invention also provides an apparatus by which to producepure granular silicon feedstock.

Some embodiments of the invention provide methods of producing puregranular silicon feedstock by continuously feeding impure silicon and astream including purified, recycled SiI₄ and I₂ into a first unit (atwo-stage fluidized-bed reactor system). The first fluidized bed maycontain an inert solid material such as silica that is maintained at aconstant temperature throughout the reactor volume and from whichemerges a vapor mixture containing the vapors SiI₂, SiI₄, I, I₂, iodidevapors of impurity elements contained in MG-Si, and entrained solidfines including unreacted MG-Si. This vapor/solids mixture istransferred without significant temperature drop into a separationvessel such as a cyclone whereupon the solid and vapor phases areessentially separated so that most of the entrained fines includingimpure silicon are recovered, directing the remaining vapor phase to afluidized bed that is maintained at a substantially lower temperatureand that initially contains some pure silicon seed particles insuspension whereupon the SiI₂ reacts both in the vapor phase to producepure solid silicon nuclei and on the pure solid silicon seed particlesto form a thin film.

The proportion of gas-phase to seed-particle silicon formation can becontrolled by varying the fluidized bed temperature, vapor composition,and quantity of seed particles. In this fashion, the fluidized bed seedparticles grow in size over time and new silicon nuclei are formed inthe gas phase to replenish the fluidized bed. The high surface area ofthe second fluidized bed allows for high production rates of silicon anda scalable method for commercial production. The excellent control oftemperature in the fluidized bed further enhances the controllabilityand optimization of the overall process.

The vapor stream exiting the second fluidized bed reactor includesmostly SiI₄, I, I₂, and SiI₃ vapor, iodide vapors of the elementalimpurities of MG-Si, and entrained pure silicon nuclei formed during thevapor-phase deposition reaction. This vapor/solid mixture is transferredto a solid/gas separator, such as a cyclone, that is held at a slightlylower temperature than the second fluidized bed so as to minimizefurther silicon nucleation in the vapor phase downstream of the secondfluidized bed. The pure silicon nuclei are trapped in the cyclone andthen returned to the fluidized bed to act as seed particles for furthersilicon deposition. Pure silicon granules in the second fluidized bedcan either be removed from the process and sold or transferred to afurther purification step in a second unit wherein another dualfluidized bed reactor system repeats the reaction/deposition processpreviously described.

In this way, the purity of the resulting silicon granules can betailored by using one or more units that alter the composition of n- andp-type impurities in the granular silicon product. The second unitdeposition fluidized bed has a silicon purity range that is 10-100 timesgreater than the silicon granules entering from the first unit. Furtherrepetition of this process can occur in further units, but economicswill generally dictate the maximum number of units.

The vapor stream exiting the gas/solid separator in the first unit isthen cooled rapidly via a heat exchanger network to a temperature ofbetween 700-800° C. to capture solids such as FeSi and then transferredto a high-temperature gas-filter system which removes these and othersolids in the gas stream.

The remainder of the vapor mixture is directed to another heat exchangersystem whereupon the temperature is reduced further to between 200-300°C. thereby condensing a portion of the vapor and this liquid/vapormixture is then introduced into a continuous fractional distillationcolumn at two locations, near the middle point of the column's lengthand near the lower part of the column. The composition of the liquid andvapor in the distillation column is mostly SiI₄. As iodine makeup isrequired in the process, a relatively pure iodine vapor/liquid stream isintroduced into the fractional distillation column closer to the uppersection of the column and is further purified within the column. In thedistillation column, the products and their output locations are asfollows: higher boiling point iodides (i.e., AlI₃, TiI₄, CI₄, and PI₃)are removed near the bottom zone; SiI₄ is removed near the middle zone;lower boiling point compounds (i.e., BI₃) are withdrawn near the upperzone; and purified iodine is removed at the top and acts as the refluxto the column thereby providing steady state operation. Pure iodineliquid and SiI₄ produced in this fashion are transferred to aliquid/solid-filter that removes any fine particles that may have beenentrained into or formed within the distillation column. Part of thefiltered liquid stream containing iodine and SiI₄ is then recycled backto the first unit fluidized bed reactor and, if the final silicon puritytarget requires it, part is directed to the second unit dual fluidizedbed reactors in such proportions that assist the further purification ofsilicon. The entire process is typically operated at a pressure onlyslightly above 100 kPa absolute pressure but the distillation column mayalso be operated under vacuum conditions.

All of the impurity iodides which are collected in either the first orsecond units (e.g., solid FeI₂, liquid AlI₃, and liquid PI₃) are mixedtogether in a liquid/solid slurry and continuously injected into aheated reactor at 400-900° C. in a dry atmosphere containing oxygen.Under these conditions, the oxides of most of the impurity elements aremore stable than their corresponding iodides and will thus release theiodine as a vapor with a conversion efficiency of 90-95 percent. Theiodine-laden gas stream is then cooled in stages to below the meltingpoint of iodine and the resultant pure liquid iodine is recycled to thepure iodine feed stream upstream of the distillation column. In thismanner, most of the iodine fed to the polysilicon production process isrecovered and reused.

Other goals and advantages of the invention will be further appreciatedand understood when considered in conjunction with the followingdescription and accompanying drawings. While the following descriptionmay contain specific details describing particular embodiments of theinvention, this should not be construed as limitations to the scope ofthe invention but rather as an exemplification of preferableembodiments. For each aspect of the invention, many variations arepossible as suggested herein that are known to those of ordinary skillin the art. A variety of changes and modifications can be made withinthe scope of the invention without departing from the spirit thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated herein and form partof the specification, illustrate preferred embodiments of the presentinvention, and together with the description, serve to explain theprinciples of the invention.

FIG. 1 is a schematic diagram of the apparatus illustrating the flow ofmaterials for the commercial production of PV- and EG-silicon.

FIG. 2 is a schematic diagram of the apparatus illustrating the flow ofmaterials for the recovery of iodine from waste streams of the processdescribed in FIG. 1.

FIG. 3 is a diagram of an integrated silicon purification or depositionsystem with dual reaction chambers.

INCORPORATION BY REFERENCE

All publications and patent applications mentioned in this specificationare herein incorporated by reference to the same extent as if eachindividual publication or patent application was specifically andindividually indicated to be incorporated by reference.

DETAILED DESCRIPTION OF PREFERABLE EMBODIMENTS

The invention provides methods and apparatus for generating one or morepolysilicon feedstock products containing tailored levels of n- andp-type impurities. Variable grades of silicon can be produced at highthroughputs and low cost with the process and apparatus disclosedherein.

Referring generally to FIG. 1, an impure silicon feed is introduced viaconduit 11 into the dense phase of a first fluidized bed 10 in Unit 1which is a dual fluidized bed system. A liquid mixture 52 includingrecycled, liquid S11 and iodine is vaporized and pumped into Unit 1 withor without the aid of an inert gas 10 a such as argon through the bottomportion of fluidized bed 10. The ratio of recycled liquid to impuresilicon feed is generally not greater than about 20:1 on a molar basis.

Fluidized bed 10 is maintained at a constant temperature throughout itsvolume in the range of 1200-1350° C. and may contain inert particlessuch as high-purity quartz to facilitate proper mixing behavior and alsoto adjust the ratio of MG-Si-to-recycle liquid 52. It shall beunderstood that the reactors or vessels provided in accordance with theinvention including those for fluidized beds 10, 20, 60, 70 in Units 1and 2 can be made of construction material typically composed of anouter metal alloy shell that provides structural strength and an innerceramic shell that is exposed to the bed particles that is resistant tohigh temperature corrosion by the halogen-bearing vapors containedtherein.

The impure silicon feed includes mostly silicon but also contains p- andn-type impurities such as boron and phosphorous and may contain a widearray of metallic and non-metallic elements. The quartz particles areproperly sized and generally remain in the dense phase and do not reactsubstantially with the vapors. The impure silicon feed particles influidized bed 10 react with vapors of iodine and SiI₄ to produce mostlythe vapor species SiI₂, SiI₄, I, SiI₃ and iodide vapors of some of theimpurity elements. This vapor stream is transported out of thedense-phase of the fluidized bed 10 to a separator 14 such as a cycloneseparator via conduit 13 under isothermal conditions. As impure siliconreacts, its particle size and mass are reduced to a point where thefluidizing medium has sufficient momentum to transport the remainingsmall particles into separator 14 from which they are removed from theprocess via conduit 16. Generally, the separator 14 is able to removeparticles as small as one micrometer in diameter or less. Largerparticles that are inadvertently elutriated from the fluidized bed canbe separated from the fines and returned to fluidized bed 10 via conduit12.

A vapor stream less the removed particles can be thus separated forfurther processing in accordance with the invention. A preferablydust-free vapor thus formed exits the separator and is transportedthrough conduit 15 to a heat exchanger 25 that lowers the temperature byseveral hundred degrees over a short distance. As the vapor is cooled inthis manner there is a tendency for silicon to be deposited either inthe gas phase as very small nuclei or as a thin film on the heatexchanger surfaces. The short residence time and flow pattern within theheat exchanger minimizes these tendencies substantially. The cooledvapor thus formed is then fed via conduit 24 into the lower section ofthe second fluidized bed 20 which is maintained at a constanttemperature in the range of 800-1000° C. Alternatively, the heatexchanger 25 may be located within the second fluidized bed 20 andimmediately upstream of the distributor plate. A further alternative isproposed wherein the second fluidized bed 20 contains a heat exchangejacket on its shell or inside the dense phase. In this case, thedust-free vapor 15 is kept at the operating temperature of the firstfluidized bed 10 until it enters the dense phase of the second fluidizedbed 20 and thereafter is cooled by the mass of particles which are at alower temperature.

The second fluidized bed 20 may include a dense-phase with initial seedparticles of high-purity silicon during the startup phase of theprocess. The fluidized bed 20 may be considered as two distinct phases—adense phase containing well-mixed suspended particles bathed in vaporand a second phase which includes vapor bubbles that travel upward in amostly vertical direction. The benefit of bubble formation is to providefor an exceptionally high mixing rate for the solids leading to uniformtemperature throughout the bed volume. The disadvantage is that bubbleslead to vapor bypassing the reaction zone and an overall reduction inproductivity. In the present case, the vapor which is contained in thebubbles tends to produce silicon mostly in the gas phase while the vaporin the dense phase tends to deposit silicon as a thin film on theparticles. The dense phase particles therefore grow during the processwhile new silicon particle nuclei are added via homogeneous gas-phasereaction of SiI₂ within the vapor phase. Because of this behavior, thereis generally no need to add new high-purity seed silicon particlesduring operation of the fluidized bed 20. As the dense phase siliconparticles reach larger sizes, it becomes useful to remove these from thefluidized bed 20 via conduit 22. These particles can either be sold 23or directed to fluidized bed 70 in Unit 2 via conduit 21 to act asstarting material for a second purification sequence similar to thatpreviously described.

Some of silicon nuclei which are formed in the gas phase of thefluidized bed 20 are transported via conduit 29 and enter the separator27 such as a cyclone wherein they are separated from the vapor and exitthrough the separator and are returned to the fluidized bed via conduit26. Separator 27 is kept at a temperature which is below about 800° C.to minimize any residual silicon formation downstream of the fluidizedbed 20.

Vapor which exits the separator 27 is transported via conduit 28 to heatexchanger 32 which reduces the vapor temperature to 500-700° C., andthen the vapor passes through conduit 31 to a high-temperature filter 30which traps fine particles of impurities such as FeSi and high-meltingpoint iodides as a filter cake. The impurities are periodically removedfrom filter 30 and collected in vessel 33.

The filtrate vapor is transferred via conduit 34 to a heat exchanger 35that reduces the vapor temperature to between 200-300° C. so as to causecondensation of SiI₄. The resultant. vapor-liquid mixture is split intotwo streams with some material pumped via conduit 36 into the middlesection and some material pumped via conduit 36 a into the reboilersection 46 of a continuous tray-tower distillation column 40. Commercialgrade iodine raw material or recycled iodine from a waste recoveryprocess (such as shown in FIG. 2) is vaporized and fed through conduit81 and condenser 80 into the distillation column via conduit 82. Thecontinuous output streams of the distillation column 40 include liquidS114 (boiling point, 288° C.) 44, liquid iodine (boiling point, 183° C.)51, and, if present, argon gas 49. At the top of the column, thecontinuous reflux 42 is primarily iodine that is condensed within thereflux exchanger unit 41. If inert gas such as argon is used in theprocess, it is cooled, separated from iodine vapors, and returned viaconduit 49 to the process Units 1 and 2 via conduits 10 a and 70 a,respectively. At the bottom of the column the continuous reboiler 46acts to control the temperature of the boilup 47 which includes mostlyhigher boiling point impurities such as aluminum iodide (b.p., 382° C.)and titanium iodide (b.p., 377° C.). During the operation of thedistillation column a quantity of impurity liquid is removed on a batchbasis from the following column trays: boron iodide (b.p., 210° C.) 43;phosphorous iodide (b.p. 316° C.) and carbon iodide (b.p., 320° C.) 45;and, aluminum iodide and higher boiling point liquid iodides 48. Theseliquid streams are then sent to a waste iodine recovery process shown inFIG. 2. Liquid iodine from the reflux loop is transported via conduit51, combined with purified liquid SiI₄ via conduit 44 and filtered in aliquid-solid filter 50 to remove any fines suspended in the liquid.These solid fines include impurity iodides and other impurity compoundsnot trapped by the gas-solid filter 30, purified silicon, and othersolid contaminants brought into the process via raw materials orproduced via corrosion action on the process vessel walls. Their removaldownstream of the distillation column ensures that solids in therecycled streams do not contaminate the process Units 1 and 2. Thefilter cake 53 is removed periodically from the process. The mixture ofpurified liquid iodine and SiI₄ is recycled through conduits 52 or 54 toUnits 1 and 2 of the process, respectively.

In Unit 2, the dual fluidized bed system is repeated as with Unit 1.However, it is noted that the size of equipment and flow rate ofmaterials in this section of the process are not necessarily identicalto those in Unit 1. Furthermore, purified silicon from this unit may becombined in different proportions with purified silicon from theprevious unit to produce a further tailoring of final silicon productcomposition of n- and p-type impurities.

Purified silicon particles at the temperature of the fluidized bed 20are continuously fed by a screw feeder or other mechanical means intothe dense-phase of fluidized bed 70 which is held at a temperature of1200-1350° C. A recycled mixture of liquid SiI₄ and iodine is heated andintroduced via conduit 54 into the bottom of the fluidized bed. Ifrequired in the process an inert gas flow 70 a may also be provided. Asin Unit 1 the ratio of recycled liquids via conduit 54 to purifiedsilicon feed via conduit 21 is generally not greater than about 20:1, ona molar basis. Fluidized bed 70 is maintained at a constant temperaturethroughout its volume in the range of 1200-1350° C. and may containinert particles such as high-purity quartz. The purified siliconparticles in fluidized bed 70 react with vapors of iodine and SiI₄ toproduce mostly the vapor species SiI₂, SiI₄, I, SiI₃, and iodideimpurity vapors of mostly B, P, and carbon, as most of the metallic andother non-metallic elements are removed by the process steps previouslyoutlined. This vapor stream is transported out of the dense-phase of thefluidized bed 70 to a separator 72 via conduit 74 under isothermalconditions. As the previously purified silicon particles react, theirparticle size and mass are reduced to a point where the fluidizingmedium has sufficient momentum to transport the remaining smallparticles into cyclone 72 from which they are captured and returned tothe fluidized bed 70 via conduit 71. Particles of previously purifiedsilicon which are too small for removal will be transported along withthe vapor from the separator 72 and via conduit 73 into heat exchanger62 that lowers the temperature by several hundred degrees over a shortdistance. These particles end up in the fluidized bed 60 as seedparticles. When this vapor/solids mixture is cooled in this manner,there is a tendency for silicon to be deposited either in the gas phaseas very small nuclei or as a thin film on the heat exchanger surfacesand the entrained particles of purified silicon. The short residencetime and flow pattern within the heat exchanger minimizes thesetendencies substantially. Alternatively, the heat exchanger 62 may belocated within the second fluidized bed 60 and immediately upstream ofthe distributor plate. The cooled vapor thus formed is then fed viaconduit 61 into the lower section of the second fluidized bed 60 whichis maintained at a constant temperature in the range of 800-1000° C. Asindicated previously, this temperature reduction can also occur withinthe fluidized bed 60 if a cooling medium is provided on the jacket orwithin the dense phase. The dense-phase of fluidized bed 60 includesseed particles of high-purity silicon during the startup phase of theprocess but does not generally require any makeup during operation forreasons previously described. As the dense phase silicon particles reachlarger sizes it becomes useful to remove these from the fluidized bed 60via conduit 63. These particles can either be sold as higher puritysilicon or directed to a third Unit and so forth for production ofhigher purity products.

Some of silicon nuclei formed in the gas phase of the fluidized bed 60are transported via conduit 66 and enter the separator 65 wherein theyare separated from the vapor and are returned to the fluidized bed viaconduit 67. Separator 65 is kept at a temperature which is below about800° C. to minimize any residual silicon formation downstream of thefluidized bed 60. Vapor which exits the separator 65 is transported viaconduit 64 to heat exchanger 32 and thus reenters the previous processsteps described.

FIG. 2 shows the process flow sheet for the recovery of iodine fromwaste streams in FIG. 1. It should be noted that the process throughputis typically only around 1/100^(th) that of the process previouslydescribed and varies with the quantity of impurities in the MG-Si aswell as the amount of equipment corrosion products that end up in thewaste streams.

Liquid streams 43, 45, 48 from the distillation column 40 in FIG. 1 arefed either individually or in combination via conduit 11 to a mixingvessel 10 at a temperature between 200-300° C. Filter cake solidsstreams 33, 53 from the process described in FIG. 1 are fed eitherindividually or in combination via conduit 12 into mixing vessel 10. Therelative proportion of streams 11 and 12 varies according to thecontaminant composition, in particular the level of boron, phosphorous,lead, arsenic and mercury. In cases where the content of theseparticular impurities is too high to achieve adequate iodine recoverythen the particular stream containing the impurity is processedindividually in the fashion described henceforth.

The liquid-solid mixture is transferred via conduit 13 to a heater 30which increases the stream temperature to around 400° C. The heatedslurry is then sprayed via conduit 31 into the lower-section of afluidized bed reactor 50 containing inert particles such as silica andoperated at absolute pressures of less than about 1000 kPa. Thefluidized bed 50 is maintained at temperatures ranging from 400-900° C.depending on the composition of impurity elements in the stream withinconduit 31. An oxygen-bearing gas is fed via conduit 21 into a towercontaining silica gel or other absorbent that reduces moisture contentdown to several parts per million. This step is beneficial to reduce thepossibility of significant amounts of water entering the process in FIG.1 along with the recycled iodine. Dried oxygen-bearing gas is thenpumped via conduit 22 into a gas-fired heater 40 which raises thetemperature of stream 41 to 500-900° C. and is then introduced into thebottom section of the fluidized bed 50 at velocities sufficient toprovide adequate mixing of the inert particles.

In fluidized bed 50 the oxygen in the gas stream 41 reacts with solidand liquid impurity iodides contained in stream 31 to produce solidoxides and a vapor containing I₂ and I. Over 95 wt % of impurity iodidesreact in this manner. The stable solid oxides of key elements that aretotally recovered at 700° C. in an atmosphere containing astoichiometric excess of oxygen are as follows: Fe₂O₃, Al₂O₃, VO₂, TiO₂,CaO, NiO, Mn₂O₃, Cr₂O₃, MgO, ZrO₂, CuO, CdO, SnO₂, Bi₂O₃, SbO₂, SrO,TeO₂, In₂O₃, CO₃O₄, and Ga₂O₃. The solid oxides of these impurityelements are produced in the fluidized bed either on a silica particlesurface or in the gas phase. The oxides of boron, B₂O₃, and arsenic,As₂O₅, can be either a solid or liquid in the fluidized bed depending onwhether the temperature is above or below about 450° C. and 600° C.,respectively. In the case where defluidization occurs due to thepresence of liquid oxides then either a two-stage oxidation isappropriate or the fluidized bed 50 can be operated in thefast-fluidization regime to effectively eliminate this potentialproblem.

If the oxides form on a silica particle then these particles grow insize over time and are eventually removed from the fluidized bed viaconduit 53. Similarly if the oxide nuclei that form in the gas phase arelarge enough to remain suspended in the fluidized bed then they too willgrow in time and eventually be removed via conduit 53. However, some ofthe gas-phase oxides form particles or liquid droplets that are toosmall and therefore can be elutriated from the fluidized bed 10 andenter a separator 51 wherein they are separated in stream 54 from thegases. The liquid oxides of boron and arsenic that enter the separator51 would be converted to solids due to the lower operating temperature.

The particle-free vapor stream which is at a temperature of 300-400° C.emerges from the separator 51 and is directed via conduit 52 into one ormore heat exchangers 60. The heat exchanger 60 reduces the temperatureof the vapor stream to around 200° C. at the outlet and then the vaporis transferred via conduit 61 to a condenser 70 that brings thetemperature of vapor to slightly above the melting point of iodine(i.e., 113° C.). In the condenser, the iodine vapor starts to condenseat around 183° C. near the entrance of condenser 70. Along the length ofthe condenser the partial pressure of pure iodine vapor is reduced from760 mm Hg at the entrance down to around 80 mm Hg near the exit. In thismanner about 80-90 wt % of the incoming iodine to the condenser 70 isconverted to liquid form. The condensed liquid iodine is then pumpedinto a liquid-solid filter 72 via conduit 71 which removes any residualfine particles not captured by cyclone 51. In addition the filter 72also removes a phosphorous oxide, (P₂O₅)₂, which is formed at around200° C. in condenser 70 from the vapor of the same composition. Thefilter cake produced in filter 72 is removed via conduit 73 and thefiltrate liquid iodine is sent to the purified iodine feed shown in FIG.1

The vapor stream from condenser 70 is then pumped via conduit 74 into acooling-water heat exchanger 80 and via conduit 81 into a refrigerationunit 90 followed by transport via conduit 91 into a cloth-filter 100operated below about 10° C. During these process steps the iodine vaporis converted to a solid form and recovered as filter cake that isrecycled to the purified iodine feed in the process of FIG. 1. Since thepartial pressure of pure iodine in the gas stream at these temperaturesis reduced to below about 0.1 mm Hg, then in this way most of theremaining iodine is recovered. The filtrate gas stream containing oxygenand small amounts of CO₂ is then transferred via conduit 101 into asplitter whereby some of the gas is recycled to the iodine recoveryprocess via 102 to stream 21 and some of the gas is discharged into theambient environment.

FIG. 3 illustrates another aspect of the invention that provides aseries of one or more silicon processing modules. Depending onparticular applications, the modules may be configured for siliconpurification, silicon deposition or other silicon processing. Apreferable embodiment of the invention provides a silicon processingmodule with dual reaction chambers. For example, a reaction chamber #1may be temperature regulated for generating a silicon iodide vaporproduct operating within a first temperature range. This chamber mayreceive one or more various inputs or feeds of silicon processingmaterials such as impure silicon feeds, inert gas/solid materials, andiodine or silicon iodide vapor mixtures, as described elsewhere herein.In a preferable embodiment of the invention, each chamber may beconfigured with apparatus to serve two roles: a silicon reactor with afluidized bed of silicon and inert materials in accordance with anotheraspect of the invention; and a separator (such as a cyclone separator)for separating silicon iodide vapor product. In alternate embodiments ofthe invention, each role may be carried out in separate equipment suchas a series of thermally controlled and fluidly connected reactors andseparators (see. FIG. 1). The silicon iodide vapor product produced inreaction chamber #1 may be passed through a temperature regulator to areaction chamber #2 operating within a second temperature range. A solidsilicon product may be thus formed in accordance with the inventionhaving a selected level of purity as a result of a temperature gradientbetween the reaction chambers, with or without the presence of siliconseed particles in reaction chamber #2. The temperature regulator may beany apparatus for creating or maintaining desired intermediarytemperature ranges. A preferable embodiment of the invention includes aheat exchanger positioned between the first and the second temperatureregulated vessels to provide selectable temperature gradientstherebetween to facilitate formation of solid silicon products. In apreferable embodiment, the silicon iodide vapor product may betransferred to a fluidized bed within reaction chamber #2 maintained ata relatively lower temperature which may contain at least some puresilicon seed particles in suspension. Silicon iodide vapor product suchas SiI₂ can be therefore allowed to react both in a vapor phase toproduce pure solid silicon product, and also on seed particles to formthin films which may be removed from reaction chamber #2. As siliconparticles reach larger sizes, they may be removed as a commercial endproduct itself, or serve as starting material or system input foranother processing or purification sequence which may be repeated asdescribed above in another successive silicon processing module. As withother embodiments of the invention herein, the illustrated siliconprocessing modules may be connected in a serial manner. Furtherprocessing or purification can be achieved by passing silicon iodidevapor and/or other inputs into successive modules.

The foregoing is considered as illustrative only of the principal of theinvention. Further, since numerous modifications and changes will occurto those persons skilled in the art, it is not desired to limit theinvention to the exact construction and operation shown and described,and accordingly all suitable modifications and equivalents may beresorted to falling within the scope of the invention as defined by theclaims which follow.

1. A method of purifying silicon, comprising: (a) inputting an impuresolid silicon feedstock and purified recycled silicon-bearing vaporsinto a first fluidized bed reactor; (b) outputting, from said firstfluidized bed reactor, an output including excess impure solid siliconand a mixture of vapors; (c) transferring said output into a solid-gasseparator; (d) separating said output into separated solids andseparated vapors; (e) removing a fines portion of said separated solidsfrom the solid-gas separator and returning a coarse part of saidseparated solids to said first fluidized bed reactor; (f) transferringsaid separated vapor to a second fluidized bed reactor; (g) initiallyproviding said second fluidized bed reactor with pure silicon seedparticles, and (h) outputting pure silicon granules from the secondfluidized bed reactor.
 2. The method of claim 1 wherein said secondfluidized bed reactor is maintained at a temperature between 800 and1000° C.
 3. The method of claim 2 wherein said first fluidized bedreactor is maintained at a temperature of between 1200 and 1350° C. 4.The method of claim 3 wherein said purified recycled vapors include SI₄and I₂ vapors.
 5. The method of claim 4 wherein said mixture of vaporsincluded SI₂, SI₄, I₂, I and iodine-bearing vapors of impurities fromsaid impure solid silicon feedstock.
 6. The method of claim 5 whereinsaid solid-gas separator is maintained at a similar temperature as saidfirst fluidized bed reactor.
 7. The method of claim 6 wherein saidsecond fluidized bed reactor is maintained at a substantially lowertemperature than the first fluidized bed reactor.
 8. The method of claim7 wherein, within said second fluidized bed reactor, SI₂ in theseparated vapor reacts while in the vapor to form pure solid siliconnuclei, and reacts with said initial silicon seed particles to form afilm of said pure silicon granules on top of said seed particles.
 9. Themethod of claim 8 wherein a mixture of SI₄, I₂, I, SI₃, andiodine-bearing vapors of impurities from said impure silicon feedstockis output from said second fluidized bed reactor.
 10. The method ofclaim 9 wherein said mixture and said pure silicon granules are providedto a second solid-gas separator.
 11. The method of claim 10, whereinsaid second solids-gas separator is maintained at a temperature lowerthan said second fluidized bed reactor to minimize silicon nucleation inthe gas phase downstream from said second fluidized bed.
 12. The methodof claim 11, wherein pure solid silicon nuclei from said secondsolid-gas separator are returned to said second fluidized bed reactor.13. A vessel for purifying silicon comprising: a temperature controlledreactor formed with an internal cavity; and a fluidized bed within saidcavity, wherein the fluidized bed is formed with a combination ofsilicon material and at least one inert material.
 14. The vessel ofclaim 13, wherein the inert material is at least one of the following:silica, argon or inert gas.
 15. The vessel of claim 13, wherein thevessel is also configured with a separator to separate vapor/solidmixtures.
 16. A method of purifying silicon comprising: inputting asilicon feedstock having an initial level of purity and an iodine basedvapor into a first temperature regulated vessel that is set to a firsttemperature; reacting the silicon feedstock with the iodine based vaporto provide a silicon vapor product; transferring the silicon vaporproduct into a second temperature regulated vessel that is set to asecond temperature; and forming a solid silicon product from the siliconvapor product within the second temperature regulated vessel having alevel of purity greater than the initial level of purity for the siliconfeedstock.
 17. The method of claim 16, wherein the first and secondtemperature controlled vessels include fluidized beds for assisting intemperature regulation.
 18. The method of claim 16, wherein thefluidized beds are formed of a silicon material and an inert material.19. The method of claim 18, wherein the silicon material consists ofsilicon seed particles.
 20. The method of claim 16, wherein atemperature gradient exists between the temperatures of the first andsecond temperature controlled vessels.
 21. The method of claim 16,wherein second temperature controlled vessel is maintained at asubstantially lower temperature than the first temperature controlledvessel.
 22. The method of claim 16, wherein the vapor product includesat lease one of the following: SI₂, SI₄, I₂, I, iodine-bearing vapors ofimpurities, recycled SI₄, recycled I₂ vapors, or a combination thereof.23. The method of claim 16, wherein the silicon vapor product isseparated using a solids-vapor separator before transfer to the secondtemperature regulated vessel.
 24. The method of claim 16, wherein thesilicon vapor product is transferred to the second temperature regulatedvessel via a heat exchanger under thermally controlled conditions. 25.The method of claim 16, wherein the solid silicon product is formed inthe presence of silicon seed particles within the second temperatureregulated vessel.
 26. A silicon processing module comprising: a firsttemperature regulated chamber for generating a silicon iodide vaporproduct operating within a first temperature range; a second temperatureregulated chamber for receiving the silicon iodide vapor productoperating within a second temperature range; and a heat exchangerpositioned between the first and the second temperature regulatedchambers for providing selectable temperature gradients therebetween tofacilitate formation of solid silicon product.